To purify an exhaust gas discharged from any type of engine or the like, a catalyst body (hereinafter referred to as a ceramic catalyst body in the present description) is utilized in which a catalyst is carried on, for example, a ceramic structure (a honeycomb structure) having a honeycomb structure. FIGS. 4 to 6 are diagrams showing one example of the ceramic catalyst body. The ceramic catalyst body having the honeycomb structure has a structure in which a catalyst layer 15 is carried on the surface of a partition wall 4 forming a cell 3 as shown in FIG. 6. As shown in FIGS. 4, 5, the exhaust gas is allowed to flow into the cells 3 from the side of one end face 2a of a ceramic catalyst body 60 (a ceramic structure 11), brought into contact with catalyst layers (not shown) on the surfaces of the partition walls 4, and discharged from the side of the other end face 2b of the structure, whereby the exhaust gas can be purified (e.g., see Patent Document 1).
In a case where the exhaust gas is purified using the ceramic catalyst body, in order to improve purification efficiency, the hydraulic diameter of the cells is decreased, and the surface area of the partition walls is increased, whereby the transmission of the components to be purified included in the exhaust gas from the exhaust gas to the catalyst layers on the surfaces of the partition walls is preferably promoted as much as possible. Moreover, to realize this, a method of increasing the number of the cells (a cell density) per unit area is employed. It is known that the transmission ratio of the components to be purified from the exhaust gas to the catalyst layers on the surfaces of the partition walls increases in inverse proportion to the square of the hydraulic diameter of the cells. As the cell density is increased, the transmission ratio of the components to be purified improves. However, pressure loss tends to increase in inverse proportion to the square of the hydraulic diameter of the cells, so that a problem occurs that the pressure loss also increases with the improvement of the transmission ratio of the components to be purified. It is to be noted that examples of a prior document concerning a countermeasure for preventing the increase of the pressure loss include Patent Documents 2 and 3.
Moreover, it is known that in a case where the rate of diffusion of the components to be purified in the catalyst layer, the purification efficiency of the ceramic catalyst body tends to decrease. Therefore, to increase the purification efficiency of the exhaust gas, it is preferable not only to increase the surface area of the catalyst layer but also to decrease the thickness of the catalyst layer on the surface of the partition wall, which is usually about several ten μm, whereby the diffusion rate of the components to be purified in the catalyst layer is preferably increased. However, in this case, the cell density and the surface area of the catalyst layer are easily increased, and the transmission ratio of the components to be purified increases, but the problem that the pressure loss increases is not solved.
Furthermore, the inflow diameter of the ceramic catalyst body is increased, and the flow rate of the exhaust gas to be circulated is decreased, whereby the pressure loss can be decreased while maintaining or increasing the purification efficiency of the exhaust gas. However, when the size of the ceramic catalyst body is enlarged, a mounting space is limited, so that a problem that it becomes difficult to mount the structure on a car remains to be unsolved.    Patent Document 1: JP-A 2003-33664    Patent Document 2: JP-A 2002-219319    Patent Document 3: JP-A 2002-301323